DESCRIPTION (Applicant's Abstract): The central premise of this application is that CD154-CD4O interactions, and interactions amongst other TNF family members (RANKL, TNF, CD3O) control the development of alloreactive CD4 and CD8+ T-cells. We hypothesize that CD4O is a crucial communication link between three cells: the CD4+ T-cell, the CD8 T-cell and the antigen-presenting cell (APC). Our application presents the use of a number of novel murine systems to study the function of CD4O, as well as other TNFR family members in the regulation of the immune response to transplantation antigens in a model of GVHD. 1. Deciphering the role of TNF family members in the controlling the expansion, differentiation and fate of alloantigen-specific CD4 and CD8 T-cells. We show that the co-adoptive transfer of alloreactive, T-cell receptor (TCR) Tg CD4+ and CD84 T-cells into a host that bears the alloantigen induces the expansion and differentiation of both Tg T-cells. The data show that expansion and differentiation of the alloreactive TCR Tg CD8+ T-cells is dependent on CD 154 expression by Tg CD4+ T-cells. The responses of the two Tg T-cell partners ultimately lead to GVHD and death of the host. This system offers a unique opportunity to understand how CD4O and other TNFR family members (TNFa, RANKL, CD3O, and Fas) control the function and fate of alloreactive T-cells. 2. Identification of the alloantigen presenting cells in GVHD. Data show that the nature of the APC is decisive in inducing T-cell anergy vs.the development of effector function and disease. While it is assumed that host dendritic cells (DCs) play a primary role in triggering the onset of alloreactive responses, this is not known. Studies are presented to identify the CD4O-bearing APC that are responsible for triggering CD4 and CD8 expansion, and for the development of GVHD. Tg expression of CD4O and/or MHC molecules in distinct APC compartments or the adoptive transfer of distinct APC populations, will be used to assess the contribution of different APCs in mediating alloreactive T-cell responses and GVHD. Our hypothesis contends that when alloantigens are presented by different APCs in vivo that the expansion, differentiation and development of T-cell effector function will be altered. 3. Function and immune deviation of the CD4 subset by blocking CD154-CD4O interactions. We hypothesize that two fundamental functions of CD4+ T-cells in the regulation of the immune response to transplantation antigens are 1) to express CDI 54, so as to mature APCs to support both CD4 and CD8 triggering, and 2) to provide cytokines to support CD4 and CD8 growth and differentiation. Our efforts will be to define the function of specific cytokines in alloreactive CD4 and CD8 responses. The second effort will be to focus on the immune deviation that is imposed on the CD4 subset when CD 154 is blocked. We hypothesize that activation of CD4+ T-cells in the absence of CD4O signaling induces deviation in the activation/differentiation of CD4+ T-cells such that these T-cells down-regulate (suppress) immune responsiveness. We will explore this phenomena using the dual Tg system presented to identify the cells and molecules that mediate this long-term unresponsiveness.